R. R. Rothenberger
Department of Horticulture
What is compost?
Compost is a
dark, friable, partially decomposed form of organic matter similar
in nature to the organic matter in the soil.
leaves, grass clippings and other garden refuse is often a problem
for gardeners, particularly in urban areas. These byproducts of
the garden and landscape can be turned into useful compost with
no more effort than it takes to bag and haul away these materials.
In many cases
the compost will serve the same function as peat moss and, thereby,
reduce gardening cost. Returning these organic materials to the
land perpetuates natural biological cycles and is an ecologically
sensible means of using organic wastes. The value of compost Good
compost consists of a small amount of soil along with decomposed,
or partially decomposed, plant and animal residues. As a soil amendment
compost improves both physical condition and fertility. It is especially
useful for improving soils low in organic matter.
matter in the compost improves heavy clay soils by binding soil
particles together, making them easier to work. Such aggregation
of the soil particles helps improve aeration, root penetration,
water infiltration and reduces crusting of the soil surface. In
sandy soils additional organic matter also helps with nutrient and
contains nutrients, its greatest benefit is in improving soil characteristics.
Therefore, it should be considered as a valuable soil amendment
and not as a fertilizer because, in most cases, additional fertilization
will be necessary to achieve maximum growth and production.
Compost is also
valuable mulching material for use around garden and landscape plants.
It may be used as a topdressing for lawns and, when it contains
a small amount of soil, as a growing medium for house plants, or
for starting seedlings.
natural decomposition under controlled conditions. Raw organic material
is converted to compost by the action of microorganisms (fungi and
bacteria). During initial stages of composting, microorganisms increase
rapidly. As the materials decompose, some kinds of microorganisms
predominate, but as they complete a certain function, they decline
while others build up and continue the decomposition.
decompose the organic materials temperatures within the pile approach
140 to 160 degrees at the center. This kills some of the weed seeds
and disease organisms in these high temperature areas. However,
in cooler sections of the heap such sterilization does not occur.
are largely responsible for the breakdown of the organic materials
require large quantities of nitrogen. Therefore, adding nitrogen
fertilizer, or materials supplying large amounts of nitrogen, is
necessary for rapid and thorough decomposition. During the break-down
period this nitrogen is tied up and not available for plant use.
It is released, however, when the decomposition is complete and
the compost is returned to the garden.
may be composted?
Many types of
organic materials can be used for composting--sod, grass clippings,
leaves, hay, straw, weeds, manure, chopped corncobs, corn stalks,
sawdust, shredded newspaper, wood ashes, hedge clippings and many
kinds of plant refuse from the garden.
It is best not
to use diseased plants from the flower or vegetable garden for composting
if the compost is later to be returned to the garden. Even though
some of the diseases are killed by the heating of compost formation,
unless the compost is frequently and thoroughly turned and allowed
to remain unused for several years, there is a chance of returning
some of these disease organisms to the garden. If diseases have
not been a problem, this precaution may not be necessary.
Also, it is
best to avoid composting weeds heavily laden with seeds. Even though
some seeds are killed during composting, if the quantity of seeds
is extremely high, many might be returned to the garden when the
compost is used and then might create an unnecessary weed problem.
may also be used in the compost heap, with the exception of grease,
fat, meat scraps and bones. These may attract dogs or other animals
and may develop an odor during decomposition. Fats are slow to break
down and greatly increase the length of time required before the
compost can be used.
Locate the heap
in a convenient but inconspicuous location. If the compost is to
be used mainly in the garden, choose a nearby location. Since the
compost pile needs to be kept moist, a convenient source of water
is helpful. Compost should never get soggy wet or the process will
stop. Therefore, don't locate it where drainage is poor and water
may stand, even for short periods.
A shaded area
is also desirable for best composting. However, don't locate compost
heaps close to trees. Tree roots are easily attracted to the loose,
moist, organic material developing at the bottom of the pile. During
the summer, roots of some trees may rapidly spread throughout the
lower areas of the heap and make the compost difficult to dig and
use. Size of the pile The size of pile needed may vary greatly with
the amount of material available. A pile should not be less than
3 feet wide and 3 feet high. Anything smaller is too small to decompose
properly. An average gardener might want a pile about 5 feet wide
by 5 feet long by 5 feet deep. Where more compost is available,
the heap should still be about 5 feet wide, for easy working, and
any convenient length.
gardener may find that if adequate compost is available, two or
three small piles provide greater flexibility than a single large
one. In this way a pile may be built and allowed to start undisturbed,
while a second pile serves as a place to put organic materials as
they accumulate. Three piles are even better, with one finishing,
one in the process of decomposition and one to which fresh materials
are being added. In this way, there is almost a continuous supply
The urban gardener
may not have enough material to build several piles or may not have
room for them. In such situations, a single, tall pile may be satisfactory.
Although not ideal, fresh materials may be added to the top and
decomposed material dug out from the bottom. This does not allow
for turning, which aids complete decomposition and heating. Nevertheless,
with limited space and material it serves a definite and useful
is possible to stack the compost in a loose pile, decomposition
is best and space is used more efficiently if it is made in some
type of bin or enclosure. Many materials may be used. The sides
should be loose enough to provide some air movement through them.
One side should open for easy turning and removal of the compost.
The heap may be round, square, rectangular or other convenient shape.
Types of enclosures Woven wire fencing (hog wire, chicken wire,
chainlink), wood slat fencing (snow-fence), cement blocks, bricks
or scrap lumber can be used to enclose a compost heap. Fencing materials
need corner supports, although a small, round heap made of slatted
fencing needs little or no support. If woven wire fencing is too
loose to contain fine materials, line the enclosure with plastic
(containing some aeration holes) to keep the pile neat and speed
Bricks or concrete
blocks may be piled without mortar, but space should be left between
some of them to allow adequate air movement through the sides. Scrap
boards are suitable for sides since there is normally enough space
between them for air movement. Lumber is gradually ruined by exposure
to the damp compost, and occasionally boards have to be replaced
as they decay. Constructing the pile Compost pile construction is
usually described in terms of layers. In actual practice such layers
are less well defined. Layering is not totally essential but provides
the quickest and most complete decomposition.
The pile may
normally be started directly on the ground. However, to provide
aeration to the bottom of the pile and improve drainage, dig a trench
across the base of the area and cover with stiff wire mesh (hardware
cloth) before the layers are begun.
Begin the pile
by spreading a 6- to 8-inch layer of organic matter over the area.
If there are different materials available, use the coarsest on
the bottom. Shredded or chopped materials decompose fastest, so
if a shredder is available, coarse organic matter should be run
through it. Materials that tend to mat, such as grass clippings,
should be placed in layers only 2 to 3 inches thick. Moisten, but
do not soak the layer of organic material.
Over the layer
of plant material, sprinkle a complete garden fertilizer such as
a 12-12-12. About 1 cup per (for each) 25 square feet of top surface
area should be adequate. An equal amount of ground limestone may
also be added to the compost unless the finished compost is later
to be used for acid-loving plants.
If fresh animal
or poultry manure is available, a 1- to 2- inch layer may be substituted
for the commercial fertilizer. Cross section of layering in compost
Next, add a
layer of soil or sod 1 to 2 inches thick. The soil contains microorganisms
that help to start the decomposition process. If there is not an
adequate source of topsoil, a layer of finished compost may be used
as a substitute for the soil.
When soil or
old compost and fertilizer are used for layering, special compost
activators or starters are not needed.
alternate the layers of organic materials, fertilizer or manure
and soil until a maximum height of about 5 feet is achieved. Firm
each layer as it is added, but do not compact it so much that air
can't move freely through it. Water each layer as it is added. Care
of the pile The compost pile must be kept moist (but not soggy)
for proper heating and decomposition. Inadequate moisture reduces
microbial activity. Excess moisture may cause undesirable decomposition
and offensive odors. During dry weather it may be necessary to add
supplemental water with weekly soaking. Covering with plastic can
reduce moisture loss and aid decomposition during extremely dry
periods. A plastic covering also protects the pile from becoming
too wet during periods of heavy rainfall.
To hasten decomposition
turn or mix the pile periodically. This will facilitate aeration
of the pile and reverse any undesirable reactions. During warm weather
the pile should be turned about monthly. In cool weather decomposition
is slower, and frequent turning is not necessary. During the winter
little decomposition occurs except in very large piles. The pile
should be turned immediately if at any time a strong ammonia or
other offensive odor is detected.
be done by slicing through the pile and inverting each slice. Where
space is available, it may be done by shifting the entire pile into
another bin, later to be moved back. The main objective of turning
is to shift materials from outer parts of the pile closer to the
center where they are better able to heat and decompose.
About a month
after starting the pile, it should be hot in the center. This indicates
that the pile is decomposing properly. Failure to heat might be
caused by too much water, improper aeration, too little nitrogen
or too small a pile.
decompose, the pile should shrink to about half of its original
height. The length of time required will vary with size of pile
and time of year. If the heap fails to decompose, it may be necessary
to restack with some new materials.
is ready for use, it should be dark and crumbly, with much of the
original identity of the materials lost. Finished compost should
have an earthy smell. If compost becomes old, it still makes a good
soil amendment, but nitrogen may be lost through volatilization
or leaching. For this reason compost should be used as soon as possible
after it is finished.
will be ready for use in four to nine months, depending on the types
of organic materials used and the climatic conditions during the
For many purposes
the finished compost is easier to use if first screened through
a 1-inch wire mesh screen to eliminate coarse or incompletely decomposed
materials. Twigs decompose very slowly and should not be added,
although a few sometimes become a part of other debris. These may
either be added to another compost heap or discarded.
Compost is suitable
to use for potting houseplants or starting many seeds. However,
since thorough sterilization is important, especially for starting
seeds, the compost should be pasteurized (sterilized) before use.
Length of time for sterilization varies with volume. Place the moist
compost or soil mix in an oven preheated to about 200 degrees F,
and heat until the center of the mass reaches a temperature of about
160 degrees F and maintains that temperature for 30 minutes. A probing-type
thermometer (meat or candy) may be helpful for determining whether
the center has been properly heated. Excess heating is not necessary
and can be harmful. Make sure you remove the soil as soon as it
has been sterilized, and allow it to cool completely before use.
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